On May 10, 1804, he married Countess Ferdinande zu Erbach-Schönberg, shortly thereafter he took over the government from his mother, but this did not last long.

As a brigadier general on the side of the Allied anti-Napoleonic troops, Ernst Casimir took part in the wars of liberation against Napoleonic France.

Due to the decisions made at the Congress of Vienna, the entire Confederation of the Rhine Principality of Isenburg initially fell to the Austrian Empire in 1815.

In 1816 Austria handed over the areas of the principality to the Grand Duchy of Hesse (Hessen-Darmstadt), which agreed with the Elector (Hesse-Kassel) on a division.

[2] In 1826 Ernst Casimir was appointed the first President of the First Chamber of the Estates of the Grand Duchy of Hesse.